1. Field of the Invention
The present invention relates to a three-dimensional memory and, more specifically, to an electron trapping based three-dimensional memory without interpage crosstalk.
2. Description of the Related Art
In order to achieve higher performance, computers are increasingly relying on parallel processing and demanding memory systems with high storage capacity and fast parallel access capability. Present memory technologies such as semiconductor memories, optical disks, magnetic disks and tapes store information across a planar surface. Due to their two-dimensional nature, these storage devices are not able to provide parallel access. As the manufacturing technology matured in recent years, the storage density of these devices (e.g., optical disks) have reached the theoretical limitation, which is proportional to 1/.lambda..sup.2.
To overcome the restrictions imposed by present two-dimensional memory devices, three-dimensional optical memories have been proposed. Since the information is stored in volume, three-dimensional optical storage devices have higher theoretical storage capacity (proportional to 1/.lambda..sup.3) than the planar memories. In addition, three-dimensional optical memory devices have the potential for parallel access. The data is arranged in two-dimensional pages, or bit planes. An entire two-dimensional page can be written or read in a single memory access operation.
The performance of a two-photon based three-dimensional memory is limited by the fatigue of the materials. In order to suppress the fatigue, the two-photon materials need to be kept at low temperature (e.g., in dry ice).
Electron trapping materials do not suffer from the above-described fatigue problem associated with two-photon material. Indeed, optical memories employing electron trapping material have been demonstrated to be capable of more than one million writing, reading and erasing cycles. This longevity makes electron trapping materials an excellent media for implementing stacked layer three-dimensional optical memories. Accordingly, three-dimensional optical memories employing electron trapping materials have been proposed. See, e.g., U.S. Pat. No. 5,163,039, assigned to the same assignee as the present invention, the disclosure of which is herein incorporated by reference.
However, as described in greater detail below, the excitation by light of one layer in a multi-layered electron trapping memory in the writing process can cause inadvertent and undesired excitation of adjacent electron trapping layers. Of course, for proper operation of a multi-layer memory, it is critically important to be able to address individual memory layers and avoid interlayer "crosstalk" when writing data into the memory device. Previously, to overcome this "crosstalk" a checkerboard encoding and a differential detection scheme has been employed. See, e.g., X. Yang, C. Wrigley and J. Lindmayer, "Three-Dimensional Optical Memory Based on Transparent Electron Trapping Thin Films, Proc. SPIE, Vol. 1773, Photonics for Computers, Neural Networks and Memories, p. 413-422 (1992).
Although the checkerboard encoding scheme has been proven to be effective in reducing crosstalk, it is desirable to provide a system which provides a three-dimensional memory without crosstalk and which does not require special encoding and decoding schemes for proper operation.